Color-based grouping and inhibition in visual search: evidence from a probe detection analysis of preview search

In four experiments, we examined selection processes in visual search using a probe detection task to measure the allocation of attention. Under preview search conditions, probes were harder to detect on old relative to new distractors (Experiment 1). This cannot be attributed solely to low-level sensory factors (Experiment 2). In addition, probe detection was sensitive to color-based grouping of old distractors and to color similarity between old distractors (Experiments 3 and 4). These effects were dissociated when the color of the old distractors changed but probe detection effects remained. Collectively, the data indicate both group-based suppression of distractors and the separate inhibition of distractor features in search.     

Visual change with moving displays: more evidence for color feature map inhibition during preview search

Preview search with moving stimuli was investigated. The stimuli moved in multiple directions, and preview items could change either their color or their shape before onset of the new (search) displays. In Experiments 1 and 2, the authors found that (a) a preview benefit occurred even when more than 5 moving items had to be ignored, and (b) color change, but not shape change, disrupted preview search in moving stimuli. In contrast, shape change, but not color change, disrupted preview search in static stimuli (Experiments 3 and 4). Results suggest that preview search with moving displays is influenced by inhibition of a color map, whereas preview search with static displays is influenced by inhibition of locations of old distractors.     

Inhibition and anticipation in visual search: evidence from effects of color foreknowledge on preview search

We present four experiments in which we examined the effects of color mixing and prior target color knowledge on preview search (Watson & Humphreys, 1997). The task was to detect a target letter (an N or a Z) that appeared along with other new letters, when old distractors remained in the visual field. In some conditions, participants were told the target's color, in others, they were not. Foreknowledge of the target's color produced large improvements in search for both baseline and preview presentations (Experiment 1). For preview presentations, the magnitude of this effect was reduced if the target shared its color with a single colored set of previewed letters (Experiment 2). Removing this similarity across the displays greatly improved search efficiency (Experiment 3). In Experiment 4, we assessed and rejected the proposal that the effects reflected the probability that the target was carried by a particular color. We discuss the results in terms of separate effects of (1) inhibitory carryover from a preview color group and (2) an anticipatory set for a known target color.     

Color grouping in space and time: evidence from negative color-based carryover effects in preview search

Five experiments addressed the role of color grouping in preview search (D. G. Watson & G. W. Humphreys, 1997). Experiment 1 used opposite color ratios of distractors in preview and second search displays, creating equal numbers of distractors in each color group in the final display. There was selective slowing for new targets carrying the majority color of the old items. This effect held when there was no bias in the preview and only the second search set had an uneven color ratio (Experiment 2). In Experiment 3, participants had foreknowledge of the target color, and effects were shown over and above those due to color biases. Experiment 4 demonstrated negative color carryover even when previews changed color. Experiment 5 showed reduced color carryover effects when previews were presented more briefly. Collectively, the results provide evidence for inhibitory carryover effects in preview search based on feature grouping.     

Visual working memory supports the inhibition of previously processed information: evidence from preview search

In four experiments we assessed whether visual working memory (VWM) maintains a record of previously processed visual information, allowing old information to be inhibited, and new information to be prioritized. Specifically, we evaluated whether VWM contributes to the inhibition (i.e., visual marking) of previewed distractors in a preview search. We evaluated this proposal by testing three predictions. First, Experiments 1 and 2 demonstrate that preview inhibition is more effective when the number of previewed distractors is below VWM capacity than above; an effect that can only be observed at small preview set sizes (Experiment 2A) and when observers are allowed to move their eyes freely (Experiment 2B). Second, Experiment 3 shows that, when quantified as the number of inhibited distractors, the magnitude of the preview effect is stable across different search difficulties. Third, Experiment 4 demonstrates that individual differences in preview inhibition are correlated with individual differences in VWM capacity. These findings provide converging evidence that VWM supports the inhibition of previewed distractors. More generally, these findings demonstrate how VWM contributes to the efficiency of human visual information processing--VWM prioritizes new information by inhibiting old information from being reselected for attention.     

Distractor ratio and grouping processes in visual conjunction search.

According to feature integration theory, conjunction search is conducted via a serial self-terminating search. However, effects attributed to search processes operating on the entire display may actually reflect search restricted to elements defined by a single feature. In experiment 1 this question is addressed in a reaction-time (RT) paradigm by varying distractor ratios within an array of fixed size. For trials in which the target was present in the array, RT functions were roughly symmetric, the shortest RTs being for extreme distractor ratios, and the longest RTs being for arrays in which there were an equal number of each distractor type. This result is superficially consistent with Zohary and Hochstein's interpretation that subjects search for only one distractor type and are able to switch search strategy from trial to trial. However, negative-trial data from experiment 1 case doubt on this interpretation. In experiment 2 the possible role of 'pop out' and of distractor grouping in visual conjunction search is investigated. Results of experiment 2 suggest that grouping may play a more important role than does distractor ratio, and point to the importance of the spatial layout of the target and of the distractor elements in visual conjunction search. Results of experiment 2 also provide clear evidence that groups of spatially adjacent homogeneous elements may be processed as a unit.     

Independence of speed and accuracy in visual search: evidence for separate mechanisms.

Data from two studies that tested children's attention using visual search for a series of targets in a complex display and a sustained-attention task waiting for signals in a similar display were subjected to Factor Analysis to explore previous indications that speed and accuracy (the number of false alarms to nontargets) on this task reflect different mechanisms. The two factors identified confirmed the separation of these two measures and also suggested that the speed factor was related to Mental Age, while the accuracy factor was related to ratings of attentional ability. It is suggested that ratings of attentional ability reflect the efficiency of executive functions, displayed in the ability to inhibit responses to nontargets in these tasks, while speed of search is related to processing speed in the nervous system. Therefore intelligence and attentional ability depend on different underlying features of the nervous system.     

Prioritized selection in visual search through onset capture and color inhibition: evidence from a probe-dot detection task

Observers performed a preview search task in which, on some trials, they had to indicate the presence of a briefly presented probe-dot. Probes could be presented on locations corresponding to old or new elements and prior to or after the presentation of the new elements. After the presentation of the new elements, probes were generally detected faster on new than on old locations, indicating prioritized selection of new elements. Prior to the presentation of the new elements, probes were detected faster on new than on old locations only when old and new elements differed in color. These results suggest that prioritized selection of new elements is mediated not by visual marking but by onset capture. Additionally, observers may apply color-based inhibition.     

Faster target selection in preview visual search depends on luminance onsets: behavioral and electrophysiological evidence

To investigate how target detection in visual search is modulated when a subset of distractors is presented in advance (preview search), we measured search performance and the N2pc component as an electrophysiological marker of attentional target selection. Targets defined by a color/shape conjunction were detected faster and the N2pc emerged earlier in preview search relative to a condition in which all items were presented simultaneously. Behavioral and electrophysiological preview benefits disappeared when stimuli were equiluminant with their background, in spite of the fact that targets were feature singletons among the new items in preview search. The results demonstrate that previewing distractors expedites the spatial selection of targets at early sensory-perceptual stages, and that these preview benefits depend on rapid attentional capture by luminance onsets.     

Search in external and internal spaces: evidence for generalized cognitive search processes

ABSTRACT— There is compelling molecular and behavioral evidence that goal-directed cognition is an evolutionary descendant of spatial-foraging behavior. Across animal species, similar dopaminergic processes modulate between exploratory and exploitative foraging behaviors and control attention. Consequently, we hypothesized that spatial-foraging activity could prime attentional cognitive activity. We examined how searching in physical space influences subsequent search in abstract cognitive space by presenting participants with a spatial-foraging task followed by a repeated Scrabble task involving search for words that could be made from letter sets. Participants who searched through clumpier distributions in space behaved as if words were more densely clumped in the Scrabble task. This was not a function of arousal, but was consistent with predictions of optimal-foraging theory. Furthermore, individual differences in exploratory search were conserved across the two types of tasks. Along with the biological evidence, our results support the idea that there are generalized cognitive search processes.     

Offsets and prioritizing the selection of new elements in search displays: More evidence for attentional capture in the preview effect

The preview effect refers to the finding that new elements in a search display are prioritized over old elements. Donk and Theeuwes (2001) suggest that this prioritization is due to bottom-up attentional capture, as new elements that abruptly appear with increases in luminance produce a preview effect whereas new elements that are equiluminant to the background do not. The present study extends this research by examining if new elements that appear from offsets, with and without luminance changes, yield similar findings to onsets. This was indeed the case, as equiluminant offsets did not produce a preview effect but offsets with corresponding increases or decreases in luminance did show the prioritization of new elements over old elements. These findings show that the prioritization of new elements is sensitive to changes in luminance but not to the direction of such changes.     

The temporal dynamics of visual search: evidence for parallel processing in feature and conjunction searches

Feature and conjunction searches have been argued to delineate parallel and serial operations in visual processing. The authors evaluated this claim by examining the temporal dynamics of the detection of features and conjunctions. The 1st experiment used a reaction time (RT) task to replicate standard mean RT patterns and to examine the shapes of the RT distributions. The 2nd experiment used the response-signal speed-accuracy trade-off (SAT) procedure to measure discrimination (asymptotic detection accuracy) and detection speed (processing dynamics). Set size affected discrimination in both feature and conjunction searches but affected detection speed only in the latter. Fits of models to the SAT data that included a serial component overpredicted the magnitude of the observed dynamics differences. The authors concluded that both features and conjunctions are detected in parallel. Implications for the role of attention in visual processing are discussed.     

Eye movements reveal distinct search and reasoning processes in comprehension of complex graphs

Hierarchical graphs (e.g. file system browsers, family trees) represent objects (e.g. files, folders) as graph nodes, and relations (subfolder relations) between them as lines. In three experiments, participants viewed such graphs and carried out tasks that either required search for two target nodes (Experiment 1A), reasoning about their relation (Experiment 1B), or both (Experiment 2). We recorded eye movements and used the number of fixations in different phases to identify distinct stages of comprehension. Search in graphs proceeded like search in standard visual search tasks and was mostly unaffected by graph properties. Reasoning occurred typically in a separate stage at the end of comprehension and was affected by intersecting graph lines. The alignment of nodes, together with linguistic factors, may also affect comprehension. Overall, there was good evidence to suggest that participants read graphs in a sequential manner, and that this is an economical approach of comprehension. Copyright © 2010 John Wiley & Sons, Ltd.     

Top-up search and the attentional blink: A two-stage account of the preview effect in search

An inefficient visual search task can be facilitated if half the distractor items are presented as a preview prior to the presentation of the remaining distractor items and target. This benefit in search is termed the preview effect. Recent research has found that a preview effect can still occur if the previewed items disappear before reappearing again just before the search items (the “top-up” procedure). In this paper we investigate the attentional demands of processing during the preview and the top-up periods. Experiment 1 found that if attention is withdrawn from the top-up stage then no preview effect occurs. Likewise if attention is withdrawn from the initial preview period then the preview effect is reduced (Experiment 2). The data suggest that the preview effect is dependent on attention being paid both to the initial display and also to the re-presentation of the old display before the search display appears. The data counter accounts of preview search in terms of automatic attention capture by new items or by inhibition of return. We discuss alternative accounts of the results, and in particular suggest an amalgamation of a temporal grouping and a visual marking account of preview search.     

Retrieval dynamics in recognition and list discrimination: Further evidence of separate processes of familiarity and recall

Two experiments tested the hypothesis that the time course of retrieval from memory is different for familiarity and recall. The response-signal method was used to compare memory retrieval dynamics in yes-no recognition memory, as a measure of familiarity, with those of list discrimination, as a measure of contextual recall. Responses were always made with regard to membership in two previous study lists. In Experiment 1 an exclusion task requiring positive responses to words from one list and negative responses to new words and words from the nontarget list was used. In Experiment 2, recognition and list discrimination were separate tasks. Retrieval curves from both experiments were consistent, showing that the minimal retrieval time for recognition was about 100 msec faster than that for list discrimination. Repetition affected asymptotic performance but had no reliable effects on retrieval dynamics in either the recognition or the list-discrimination task.     

Perceptual grouping and attention in visual search for features and for objects

This article explores the effects of perceptual grouping on search for targets defined by separate features or by conjunction of features. Treisman and Gelade proposed a feature-integration theory of attention, which claims that in the absence of prior knowledge, the separable features of objects are correctly combined only when focused attention is directed to each item in turn. If items are preattentively grouped, however, attention may be directed to groups rather than to single items whenever no recombination of features within a group could generate an illusory target. This prediction is confirmed: In search for conjunctions, subjects appear to scan serially between groups rather than items. The scanning rate shows little effect of the spatial density of distractors, suggesting that it reflects serial fixations of attention rather than eye movements. Search for features, on the other hand, appears to independent of perceptual grouping, suggesting that features are detected preattentively. A conjunction target can be camouflaged at the preattentive level by placing it at the boundary between two adjacent groups, each of which shares one of its features. This suggests that preattentive grouping creates separate feature maps within each separable dimension rather than one global configuration.     

Psychophysiological evidence for continuous information transmission between visual search and response processes.

This study was designed to test whether information transmission between the perceptual and motor levels occurs continuously or in discrete steps. Ss performed visual search across nontargets that shared visual features with one of two possible targets, each assigned to a different response. In addition to reaction time, psychophysiological measures were used to assess the duration of target search and the onset of central and peripheral motor activity. Nontargets sharing features with a target selectively activated the response associated with that target, even when it was not present in the display. This suggests that information transmission to the motor level can consist of fine-grained visual information and that visual search and response selection occur in parallel.     

The role of color in visual search in real-world scenes: evidence from contextual cuing

Because the importance of color in visual tasks such as object identification and scene memory has been debated, we sought to determine whether color is used to guide visual search in contextual cuing with real-world scenes. In Experiment 1, participants searched for targets in repeated scenes that were shown in one of three conditions: natural colors, unnatural colors that remained consistent across repetitions, and unnatural colors that changed on every repetition. We found that the pattern of learning was the same in all three conditions. In Experiment 2, we did a transfer test in which the repeating scenes were shown in consistent colors that suddenly changed on the last block of the experiment. The color change had no effect on search times, relative to a condition in which the colors did not change. In Experiments 3 and 4, we replicated Experiments 1 and 2, using scenes from a color-diagnostic category of scenes, and obtained similar results. We conclude that color is not used to guide visual search in real-world contextual cuing, a finding that constrains the role of color in scene identification and recognition processes.